Uninterruptible power supplies or systems (UPS) are coming into common use to backup the power supplied from the commercial power system to critical loads such as computers, telephone systems or medical equipment. In case of a black-out or disturbance on the commercial power system, the UPS takes over the supply of power to the critical load during the interruption. Although various designs are used for UPS systems, they typically provide power during black-outs from a storage battery through an inverter to the critical load during the time that the power system is down.
Because most UPS systems utilize a battery to provide the stored energy to power the critical load during black-outs, the run time for UPS operation is limited by the storage capacity of the battery. Thus, the most straightforward present way of extending the UPS run time is simply to use additional batteries. However, this approach has several difficulties. One is cost, since the number of batteries required is generally proportional to the desired run time. Moreover, increasing the number of batteries presents additional problems. Battery connections must be kept tightened and free of corrosion, which increases the maintenance cost. To achieve optimum battery life, the batteries must be maintained fully charged and at the proper temperature. Battery storage rooms often require special ventilation. Multiple strings of batteries often become unbalanced as they age. Large number of batteries also occupy valuable floor space and their weight may cause structural loading problems. Furthermore, batteries commonly require replacement in three to five years, which may be substantially less than the life time of the UPS system as a whole, thus increasing the cost of the system over its lifetime.
An alternative to the use of additional batteries to provide auxiliary power to a critical load during long blackouts has been the use of a gasoline fueled alternating current (AC) generator (alternator). AC generators have traditionally been used to provide backup power to critical loads, including entire buildings, during long power outages in the commercial power system. To avoid even a momentary interruption of power to critical loads, a UPS may be used to supply the critical loads for a short period of time, after which the gasoline engine of the AC generator is started, allowing the generator to take over the supply of power to the load. This may be accomplished by disconnecting the UPS from the load and directly connecting the AC generator to the load during prolonged power outages. In some UPS systems, the AC generator is switched in to the input terminals of the UPS to substitute for the failed line power during prolonged power outages. Such a connection of the AC generator is most typically utilized in double inverter UPS systems in which the critical load is constantly supplied with power from the UPS inverter even during normal operation.
The utilization of an auxiliary gasoline engine driven AC generator to provide the backup supply power is useful but has certain disadvantages. Commonly, the gasoline engines which power the generator will not start when needed. The unreliability of the gasoline engine is particularly a problem where the generator is at a remote location where frequent preventive maintenance is not possible or convenient. Because such standby generators are only occasionally started to supply power to the load during power outages, the condition of both the engines and generators can deteriorate. For example, engines which run only infrequently may be subject to corrosion problems caused by condensation, lack of lubrication on bearing surfaces, and other problems associated with long term idleness.
If AC generators are utilized to provide power to a conventional double conversion UPS, the generators must typically be oversized by a factor of 2.5 to 3 times. Such oversizing is required because of the high crest factor load created by the double conversion of the AC power to DC power back to AC power. For example, a 10 kilowatt (KW) UPS generally will require a 25-30 KW AC generator. Additional costs are incurred for the installation and the cost of an automatic transfer switch.
If the critical loads are to be switched from the UPS to the AC generator, once it is up and running, the generator must be phase locked and synchronized with the output of the UPS so as to maintain a smooth transition. This transition is often difficult unless sophisticated and costly electronics are utilized. Such electronics adds to the cost and complexity of the system, but a poor power transition can disrupt the flow of power to a particularly critical load, such as a computer, and cause it to malfunction.
A problem associated with the use of AC generators connected directly to the load is that the AC generator output is often unstable, which results in additional system problems. To stabilize the frequency of the output, the engine/generator set is often oversized. Mechanical or electronic speed governor systems are required, which add cost and complexity to the system and reduce reliability. If the load on the AC generator changes in magnitude or phase angle, the output waveform from the generator is often distorted. These distortions can affect the operation of the more sophisticated UPS models, which examine sags in their input waveform to anticipate outages and serious under-voltages. Such cyclic load-induced variations in generator waveforms can cause such UPS systems to revert to battery operation frequently, even when battery power is not actually necessary to support the load. The result is battery depletion and reduced backup capabilities.
AC engine/generator sets have also been utilized as auxiliary power supplies for other applications where a UPS is not necessary, for example, in telephone systems where the batteries which provide power to the telephone networks are charged with rectified power from the AC power lines. Auxiliary engine/generator sets may be utilized to provide the charging power to these batteries during power system failure. However, such engine/generator sets have been subject to the same types of reliability and maintenance problems discussed above.